3.1. Calculation of solar heat flux on the absorber tube
The SolTrace software is utilized in order to calculate solar heat flux distribution on the absorber tube. In this
application two objects should be clarified. How to divide the calculation time into smaller intervals in which the
variation of direct normal irradiance (DNI) can be neglected? And what is the value of DNI for each time intervals?
To answer these two questions, Daneshyar [9] solar radiation model is utilized. Daneshyar model is able to calculate
the time variation of DNI in each day. Fig. 2a indicates the results of Daneshyar model at the two equinoxes and
also at the two solstices for the city of Shiraz (Latitude: 29.6° N, Longitude: 52.53° E), Iran. It should be mentioned
that for these four days the cloud factor in Daneshyar model is considered to be zero and consequently with this
assumption, from view of solar radiation the upper limit is considered.